Modern gas and steam turbine plants are designed for high steam temperatures, which subject the steel-based pipe material currently available to loading up to the permissible limits. Furthermore, these plants additionally demand a considerable degree of operational flexibility, e.g. daily rapid start up, and this leads to additional considerable loading on the pipe material.
In order to ensure that the steam temperature at the outlet of the waste heat steam generators (WHSG) does not exceed the maximum permissible or the procedurally required temperature in all possible operating states, steam temperature control devices are provided in the superheater parts of the waste heat steam generators. These steam temperature control devices operate on the basis of the mixing principle, i.e. cold medium is admixed to the hot medium to be controlled (usually steam).
In the field of power plants, the term “injection cooler” has become established for this type of steam temperature control.
The usual design and arrangement of the injection cooler systems in waste heat steam generators of gas and steam turbine plants leads to a high level of loading on the injection coolers and the following pressure system, which consequently can lead to damage here.
Typical instances of damage are, for example:                cracks in the injection nozzles of the injection coolers        demolition of the injection quill of the coolers        cracks and warpage in the protective shroud of the coolers        cracks in the pipelines        erosion at the injection points in downstream sections        
This damage can be attributed substantially to two reasons: thermal shock and droplet erosion.
The probability of damage increases greatly with rising steam temperatures and the flexibility of the gas and steam turbine plants which is required by the market.
In the presently common design of the injection system, it is irrelevant whether a drum boiler or a forced-circulation boiler is present. The injection water is withdrawn between a feed water slide and a feed water control valve and conducted to the injection coolers via an injection water line. In order that the injection water does not cool excessively in the case of inactive injection, provision is made of a circulation line, which recirculates the injection water back to the preheater part of the waste heat boiler downstream of the feed water control valve (“injection water recirculation”).
The admission pressure required for atomizing the water in the injection cooler is ensured by virtue of the fact that there is a pressure loss between the feed water control valve and the injection point. This pressure difference is also the drive for the heat retention system described with a circulation line.
In the case of this arrangement, there is a very large temperature difference (>300 K) between the injection medium (feed water) and the steam at the injection point. The risk of thermal shocks is increased. In addition, the very cold injection water, compared to the steam, has the effect that the section required for droplet dissolution in the flow of steam downstream of the injection cooler has to be extended to a significantly greater extent in order to counteract the risk of droplet erosion.
A possible improvement in the conditions at and around the injection region can be achieved by virtue of the fact that the injection water is withdrawn at higher temperatures. It is thereby possible for both the risk of thermal shocks and the problem relating to droplet erosion to be improved significantly.
In order that the admission pressure required for the injection water can continue to be maintained, the control valve likewise has to be moved.
Alternatively, the use of a valve referred to as a “pinch valve” is possible, instead of displacing the control valve. In this respect, an additional throttle valve is inserted into the main line (feed water line) of the economizer system (feed water preheating system), in order to provide the admission pressure required for the injection.
Both possibilities have a significant disadvantage, however. The entire pressure part of the waste heat steam generator up to the control valve or pinch valve has to be designed for considerably higher pressures (pump zero delivery head). This results in considerably higher costs caused by the greatly increased use of material and a reinforcement of the supporting structures caused by the considerably higher weights.